The present invention relates to a thin film field effect transistor or other thin film semiconductor device, and to a process for producing the same. More particularly, it relates to an improved self-aligned, staggered configuration which allows the underlying insulator films to serve as anti-reflective elements to maximize the transmission of light through the thin film structure during processing.
Thin film field effect transistors, or thin film transistors, are well known in the art. One of their primary uses is in the area of large area flat panel displays, such as liquid crystal displays (LCDs). In such a display, an array of display elements may be interconnected together with thin film transistors via horizontal and vertical bus bars. For example, the gates of one row of thin film transistors are connected to a horizontal bus bar while the sources are connected to the vertical bus bars. When a voltage is applied to a predetermined horizontal bus bar and to a predetermined vertical bus bar, the gate source and drain which form a particular thin film transistor are energized. In the case of an LCD, that part of the liquid crystal which corresponds to the energized transistor becomes transparent.
Amorphous silicon thin film transistors have great potential for low cost and large area liquid crystal displays. Several known processes for fabricating self aligned thin film transistors are particularly attractive. Alignment requires expensive, and in the case of very large size panel displays, presently unavailable tooling. A self aligned process does not require alignment in successive lithographic steps. However, a major drawback of the known self-aligned processes of manufacturing thin film transistors is that they require the exposure of a photolithographic resist through the thin layer of amorphous silicon. Due to the high reflectance and high absorption coefficient of amorphous silicon, long exposure times are required and the contrast between exposed and unexposed areas is reduced.
The reflectance and absorption losses in the structure limit the repeatability of thin film structures manufactured by known methods. Also, the thickness of amorphous silicon must be thinner than the minimum sufficient for device operation to allow the transmission of enough lithographically active light to the photoresist layer. A second amorphous silicon deposition and accompanying photo and etch steps are required after the formation of the source and drain electrodes to provide the necessary thickness of silicon for an operational device.